In recent years, inner focusing type lenses have come to constitute the mainstream in integrated cameras for consumer use. To achieve more compactness and make possible shooting of an object just in front of the lens, an inner focusing type lens system has a configuration in which the moving locus of the correcting lens is stored in advance in a microcomputer as lens cam data, the correcting lens is driven according to the lens cam data and focusing is also accomplished with the correcting lens instead of mechanically linking the correcting lens and a variable-magnification lens with a cam, resulting in advantages of a lower price, a simplified system and a more compact and lighter lens tube.
FIG. 6 shows a configuration of an inner focusing type lens system according to the prior art.
In FIG. 6, reference numeral 901 denotes a fixed first lens group; 902, a magnifying second lens group (zoom lens); 903, a diaphragm; 904, a fixed third lens group; 905, a fourth lens group (focusing/compensating lens) combining the focusing function with the so-called compensating function, i.e. the function of compensating for a shift in the focal plane due to a variation in magnification; and 906, the image plane.
As is known to the persons skilled in the art, since the focusing/compensating lens 905 combines the compensating function with the focusing function in a lens system illustrated in FIG. 6, the position of the focusing/compensating lens 905 for focusing on the image plane 906 varies with the distance to the object even if the focal distance is unchanged. In FIG. 7, the positions of the focusing/compensating lens 905 for focusing on the image plane 906 when the distance to the object is varied at each focal distance are continuously plotted. During a variation in magnification, the locus shown in FIG. 7 is selected according to the distance to the object, and clear zooming can be achieved by shifting the focusing/compensating lens 905 following the locus.
In such an inner focusing type lens system, when it is desired to manually drive the zoom lens 902 or the focusing/compensating lens 905, the usual practice is to provide a manipulating member which does not come into contact with the lens group, electrically detect the manipulated quantity of the manipulating member, and control a lens driving actuator or the like by a quantity corresponding to that manipulated quantity to shift the lens accordingly. In this connection, a system of shifting the zoom lens 902 or the focusing/compensating lens 905 by fitting an encoder to the lens tube and electrically detecting the direction and speed of the rotation of this encoder is proposed with a view to enabling the user to experience more of the feel of operating a front lens focusing type of a camera for professionals.
The configuration of the encoder will be described in detail below with reference to FIG. 8A and FIG. 8B wherein a zoom ring not mechanically connected to the zoom lens 902 and a focusing ring not mechanically connected to the focusing/compensating lens 905 are provided. FIG. 8A shows an external view of the encoder, and FIG. 8B, an enlarged view of a portion 605 surrounded by broken lines in FIG. 8A together with a ring rotation detecting circuit.
In FIG. 8A, reference numeral 601 denotes the focusing ring of a rotary type encoder to be snapped onto the lens tube; 602, the comb-shaped structure of the encoder having a light reflective portion and a light transmissive portion; and 603 and 604, ring rotation sensors each having a light projecting portion 606 and a light receiving portion 607, varying in the state of output signals depending on whether a reflected light from the comb-shaped structure 602 is received or not.
When the encoder of such a structure is rotated, the output signals of the ring rotation sensors 603 and 604 vary as shown in FIG. 9A or FIG. 9B. The positional relationship between the ring rotation sensors 603 and 604 is so determined that there be an appropriate lag between the phases of the two output signals; the rotating speed is detected according to the cycle of the variation of the output signals and the rotating direction, by the positional relationship between the two signals. Thus, if the output waveform shown in FIG. 9A is what is seen when the focusing ring 601 is operated in the forward rotating direction, that in FIG. 9B is what is seen when it is operated in the backward rotating direction. By taking in these output signals of the ring rotation sensors 603 and 604, the driving direction and speed of the lens are determined according to the state of those signals. Incidentally, T in FIG. 9A represents the length of time taken to shift the comb-shaped structure 602 by one tooth (a half cycle).
This image pickup apparatus, in spite of operating with an inner focusing type lens system, is enabled to perform power zooming and power focusing while retaining the feel of manipulation like a front lens focusing type by having the encoder shown in FIG. 8A and FIG. 8B and driving a lens actuator, such as a stepping motor according to the rotation of the focusing ring 601.
However, the above-described image pickup apparatus having an inner focusing type lens system according to the prior art, when the lenses are to be manually driven, a current in the order of tens of mA has to be let flow in order to secure a sufficient quantity of light emission because of the use of the ring rotation sensors 603 and 604 for the encoder. Nowadays, great importance is attached to the duration of battery charge and the requirement for power saving in the camera itself is increasingly stringent, the aforementioned amperage is too high to ignore, but the encoder shown in FIG. 8A and FIG. 8B embodies no particular consideration for power control, but consumes power wastefully.
Thus, while manual operation of lenses is an action only for shooting, if power is supplied to the camera unit during viewing, the encoder will also consume unnecessary power, which is undesirable, and the manual focusing function, which needs no manipulation when the auto-focusing (AF) function is at work, poses a particular problem to energy saving. Conversely, even in the AF mode, the user may have an intention to check the focusing when the focusing is locked after AF by half pressing of the photo key in still picture taking for instance, and it is preferable for the focusing ring 601 to remain effective.
Thus, the image pickup apparatus according to the prior art embodies no consideration for power saving while making possible manual operation in any scene when the photographic situation demands its use.